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Post subject: 1967 Bassman AB165 Fixer-Upper Problems
Posted: Thu Aug 03, 2017 4:01 am
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Location: Rochester, VT
Hi folks -- first post here and of COURSE it's digging for some answers! I repair vintage/antique guitars for a living but do minor tube amp repairs on the side for my own enjoyment and gear-pile -- but recently a friend of mine has been in "a situation" and I offered to spruce-up his '67 Bassman on the house. It's a Blackface but it's an AB165 model so it's approaching some of the aspects of a Silverface circuit.

When we initially plugged it in, I noticed a bit of current to the panel/jack as you'd might expect with a non-modded 2-prong of this age, and the volume was nowhere to be found. With knobs turned all the way up (and only on the guitar channel), we could get a whisper out of it.

So, naturally, I swapped the set of tubes -- hoping that was most of it. That brought it up in volume to something comparable to a 1-2w practice tube amp. I'm sure some or most of the original tubes are alright, but haven't had time to swap them in and see which ones.

After that I converted it to a 3-prong receptacle (replaced the "accessory" outlet, thankee-gods), removed the "death cap," and replaced the filter caps and any of the "other big brown electrolytics" that were obvious with F&T or Sprague comparables.

The amp is safe, now, but only the guitar channel's working so far and power (with the original 2x12" cab) is roughly comparable to a 10w (but, ya know, fuller) and it doesn't push towards break-up at all. Hrum!

I've done almost the same work as can be found on this guy's blog post:

http://ppamps.blogspot.com/2012/12/ab16 ... ation.html

...but I haven't replaced any of the ceramic caps (including the coupling cap) on the main "board" or control-panel "board."

Anyone have suggestions on where to go next? I'm thinking I should probably replace the coupler cap and start replacing other ceramic caps but I'm not sure what to do with the red "gumdrops" and don't reaaaaaally want to go after the "blue molded" fellas. This isn't my first time doing the "obvious cap job" and removing old electrolytics from the equation, but it is the most complicated circuit otherwise that I've dealt with so far. I'm more of a 5-10w guy with minimal controls and that's what I buy for my own enjoyment and have more experience with.

Thanks in advance!


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Post subject: Re: 1967 Bassman AB165 Fixer-Upper Problems
Posted: Thu Aug 03, 2017 6:05 pm
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If you feel comfortable doing this, get a layout and check some voltages. Pay attention to the ones marked VAC and set your meter accordingly. The others are DCV. How are the resistors (screen and grid) on the back of the 6L6 sockets? You can do the "wooden stick" tap test on the board to check for bad solder joints with a signal present.

You do realize what kind of voltage you're dealing with, right? People may be hesitant to reply because they don't want to be responsible for someone's electrocution. Just asking.


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Post subject: Re: 1967 Bassman AB165 Fixer-Upper Problems
Posted: Thu Aug 03, 2017 6:49 pm
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I second the voltage checks versus values on the schematic. Also, do you have access to a tube tester? It sounds like one or more of the tubes in the gains stages are weak or bad.

Good luck! Great amp. I redid my SF AB165 back to blackface, too. Watch the polarity of the output tranny and global feedback network!

:)


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Post subject: Re: 1967 Bassman AB165 Fixer-Upper Problems
Posted: Sat Aug 05, 2017 3:04 pm
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Location: Coastal Bend, Tx.
A chassis picture would help significantly.
Otherwise, precede w/ the suggested use of the desired amp schematics and voltage readings.
Start w/ and unloaded test of the rectifier/PSU voltages and work towards the input adding one section of tubes at a time(turn off & cool before adding tubes). This will help identify which section may have problems.

With no tubes, plug in and test for wall voltage "AC" at power switch(black wire120vac+).Turn on amp/off standby, check fuse and lamp(6.3vac). Check rectifier tube socket pin 4 & 6 for AC voltage from PT and pin 2 & 8 for heater voltage(5vac).
Ensure the speakers are properly connected. Then turn it off and add the rectifier tube, when you repower the amp you should now have power to the PSU capacitors and dropping resistors, OT and pins of the tubes those pieces are directly connected to.
If all the test points are near or relative to the schematics proceed to the next section(output tubes) then (PI). And repeat as necessary until you have all the tubes in place and or have identified a section that is not good.


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Post subject: Re: 1967 Bassman AB165 Fixer-Upper Problems
Posted: Sat Aug 05, 2017 5:44 pm
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Rock Star
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Here's is one photo of my finished Bassman reverted back to AA864, from AB165. Photo is of global feedback resistor, PI tail section, and bias pot wiring.

Image

Image

Image

Tone Lizard has a good write-up on PI tails, local and global feedback issues. :)

http://tone-lizard.com/bassman-mods/


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Post subject: Re: 1967 Bassman AB165 Fixer-Upper Problems
Posted: Thu Aug 10, 2017 6:30 am
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Thank you, guys, these are all good suggestions! I'll let you know how it goes as I procede.


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Post subject: Re: 1967 Bassman AB165 Fixer-Upper Problems
Posted: Thu Aug 10, 2017 8:04 am
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Professional Musician
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Joined: Tue Nov 27, 2012 6:56 am
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Location: Coastal Bend, Tx.
I should have mentioned that the unloaded (no tubes) test will allow you to preset the bias voltage and test check the diode as well.
After powering on the amp, you will have AC voltage to the bias supply diode. Probably around +60vac +/- 10% and will have - DC voltages at the actual bias pot and pin 1 of each output tube, adjusting the pot fully left and right will give you your bias range. Ensure you have good resistors and replace as necessary before installing good output tubes.


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